The present invention pertains generally to systems and methods for radar mapping. More particularly, the present invention pertains to systems and methods for radar mapping using synthetic aperture radar (SAR) in stripmap mode. The present invention is particularly, but not exclusively, useful for creating an SAR stripmap having a relatively large range swath.
Synthetic aperture radar (SAR) is a radar system that generates the effect of a long antenna by signal processing rather than by the actual use of a long antenna. Specifically, the long antenna is synthesized through the motion of a small antenna relative to the target with either the antenna, the target or both moving. The synthesized antenna length (i.e. aperture length) is given by the trajectory traversed by the small antenna relative to the target during the time the echoes received by the antenna are processed. The relative movement between the antenna and target produces frequency shifted return echoes (i.e. Doppler shifts) that can be processed by the SAR to produce radar images having excellent cross-range resolution. The excellent cross-range resolution allows SAR to produce images that are somewhat similar in appearance to optical photographs and is the main reason why SAR has become an especially effective tool for all-weather ground mapping.
SAR is typically used in either spotlight mode when fine resolution is desired or stripmap mode when mapping of a larger area is desired. In spotlight mode, a single image is made of a fixed position on the ground using a single synthetic aperture to produce the image. On the other hand, in stripmap mode, the radar continuously images at a fixed range from a moving platform such as an aircraft. As the platform moves, a linear series of contiguous images is made corresponding to a series of synthetic apertures. Stripmap mode is often used when searching for targets of opportunity or when a map of a large area is required.
Heretofore, SAR mapping in stripmap mode has generally been accomplished by imaging with the SAR radar at a substantially constant depression angle, xcfx86. This results in a mapping of a ground strip having a range swath that is generally limited by either the elevation beamwidth of the radar beam (beam limited), or for a given resolution, the number of pixels that the radar system can accommodate (pixel limited). In greater detail, for a given resolution, range swath is generally pixel limited at small depression angles, xcfx86, and beam limited at larger depression angles, xcfx86.
During stripmap imaging at a constant depression angle, xcfx86, the section of the ground strip that is imaged during a specific aperture must be illuminated by the radar source. Additionally, to obtain a preselected resolution, a specific minimum aperture time (corresponding to a minimum distance the platform must move during imaging of the section) is required to map the section during the illumination. When a relatively coarse resolution is acceptable, the illumination time can greatly exceed the required aperture time resulting in a significant portion of the radar system""s imaging capability going to waste.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of SAR mapping a ground strip having a relatively large range swath. It is another object of the present invention to provide systems and methods for SAR mapping a ground strip having a relatively large range swath that can be easily implemented on existing radar systems with only minor modification to the existing radar system. Still another object of the present invention is to provide systems and methods for producing an SAR map having a uniform resolution of a ground strip having a relatively large range swath. Yet another object of the present invention is to provide systems and methods for SAR mapping a ground strip having a relatively large range swath which are easy to use, relatively simple to implement, and comparatively cost effective.
The present invention is directed to a system and method for producing a synthetic aperture radar (SAR) mapping of a ground strip from a moving platform, such as an aircraft. As detailed further below, the present invention allows the mapping of a coverage area that is not necessarily limited by the radar beam shape or the number of range pixels used by the radar. For the present invention, the ground strip is divided into portions which are sequentially mapped using SAR and the resultant maps are assembled together (i.e. mosaicked) to produce one contiguous SAR map of the ground strip.
An exemplary ground strip is generally rectangular shaped having a size that is defined by a range swath in the range direction (i.e. the direction normal to the direction of platform movement) and an overall length in the azimuthal direction (i.e. the direction that is parallel with the direction of platform movement). As such, the ground strip is bounded by a near-range edge and a far-range edge, with both edges extending substantially parallel to the direction of platform movement. During SAR mapping, a radar beam that subtends a substantially constant azimuthal beam angle is used, and accordingly, the beam has a substantially constant azimuthal beamwidth, W, at the near-range edge.
As indicated above, the ground strip is divided into portions that are sequentially mapped. In greater detail, the ground strip is divided into rectangular portions that make up a tile-like pattern. The tile-like pattern can be characterized as having a plurality of columns, with each column extending from the near-range edge to the far-range edge and containing two or more portions. Each column has a column length in the azimuthal direction that is equal to the near-range beamwidth, W. Accordingly, each column (and thus each portion) has a substantially equal length. The width of each portion measured in the range direction, however, is not equal for all portions. Instead, as detailed further below, the width of each portion increases with increasing distance (i.e. range) from the platform.
The ground strip is mapped by sequentially mapping each column, and each column is mapped by sequentially mapping each portion within the respective column. As the platform moves, each column along the ground strip is sequentially illuminated by the radar. More specifically, each column is illuminated while the platform moves through a distance Lillum that is equal to the distance W (i.e. the near range beamwidth). During illumination of a column, the portions within the column are sequentially SAR mapped by altering the depression angle, xcfx86, of the radar beam.
For an exemplary column having N portions (p1, p2 . . . pN), each portion in the illuminated column is SAR mapped using a respective SAR aperture length (l1, l2, . . . lN). For example, the aperture length l1 corresponds to the distance the platform moves while the portion p1 is mapped. It follows that the sum of aperture lengths for the column must be less than or equal to the distance the platform moves during illumination of the column (l1+l2+ . . . +lNxe2x89xa6Lillum). In one implementation, the aperture length is increased as the depression angle decreases to allow all portions to be mapped with the same resolution (l1 less than l2 less than  . . .  less than lN) where l1 is the aperture length for a portion lying along the near-range edge.
During the sequential mapping, portion maps are stored in memory until all portions have been mapped. Once all portions in the ground strip have been SAR mapped, the stored maps are mosaicked, for example using image processing software, to produce a single SAR map of the ground strip.